The present invention relates to disc brakes for vehicles, and in particular to a system and method for compensating for wear between a brake disc and brake pads in disc brakes such as air-operated disc brakes utilized on commercial vehicles.
In pneumatically-actuated disc brakes (also known as “air disc brakes” or “ADB”), such those used in commercial vehicle applications, a pneumatic actuator supplied with compressed air from the vehicle fastened to an inboard side of the brake caliper has a pushrod which contacts a pivoting brake actuating lever inside of the caliper.
An example of such a brake is shown in FIG. 1. In this cross-section view, a pneumatic brake actuator 12 includes a service brake section 11 fastened to the housing 2a of a brake caliper 3. The service brake section includes a diaphragm-driven rod 10, the end of which is received in a socket of the rotary brake actuation lever 9 of the brake caliper's brake actuation mechanism. The lower portion 8 of the rotary lever 9 is located in eccentric bearing arrangement, arranged so that as the lever 9 rotates (guided by bearing 7 at a rear portion of the caliper), the lever pivot 13 advances in the direction of the brake disc 1. As the pivot 13 advances, a brake actuation tappets 14 are advanced toward brake application side brake pad 19 into contact with the inboard friction surface of brake disc 1. Once in contact with the brake disc, backing 2b of the brake caliper 3 (which straddles the brake disc 1) slides parallel to the brake disc rotation axis to press the reaction side brake pad 20 into contact with the brake disc 1. Such sliding caliper brakes are axially displaceable on guides, such as the guide sleeve 6 and guide pin 4 arrangement in FIG. 2.
Commercial vehicle air disc brakes are often provided with two brake application tappets arranged in parallel, being carried by a “bridge” block 17 that is centrally driven by the rotary lever 9. In addition, they may be provided with devices such as a return spring 15 intended to ensure that the brake pads are not being biased against the brake disc by the brake application tappets when the brake is not being applied.
Typically, such ADB brake actuation mechanisms incorporate wear adjusters designed to compensate for thinning of the brake pad friction material as the brake pads wear. The wear adjusters are configured to incrementally advance the brake actuation tappets toward the brake disc as needed to help maintain a consistent desired “air gap” between the brake disc and the brake pads when the brake is not applied. Maintaining a desired air gap helps ensure the brake pads will consistently quickly engage the brake disc when the brake is applied, and to ensure that the point at which the brake pads contact the brake disc during brake application will continue to occur before the rotary lever 9 reaches its end of travel (the end-of-travel position is illustrated in FIG. 1 by the interrupted outline of rotary lever 9).
Air disc brake wear adjusters have traditionally been complicated arrangements, due to the need to synchronize the adjustment movements of both brake application tappets, and the need to provide a readily accessible and reliable way to allow the brake application tappets to be backed out away from the brake disc, for example during brake service in which the brake pads are to be replaced (i.e., when the wear adjusters are near the end of their available travel). The wear adjuster mechanism in FIG. 2 includes spindles 24 which engage internally fluted tappets 14. As the spindles are rotated, the tappets are axially advanced toward the brake disc 1. In this known embodiment, the wear adjuster spindles are connected by sprockets 32 and a chain 30 to one another for adjuster synchronization and to a mechanical adjustment device. In this wear adjustment example, one of the wear adjuster spindles 24 is provided with a rear extension 61 which protrudes through the rear wall of the caliper body 2. After removal of an environment exclusion cap 37, a technician may manually rotate the spindle in a brake release direction to cause the spindles (synchronized via chain 30) to withdraw the brake application tappets 14 away from the brake disc to provide enough space for insertion of new brake pads.
The prior wear adjustment arrangements are complicated and costly. These and other problems are addressed by the present invention, which provides a simpler and lower-cost approach to compensating for brake pad wear in such brakes. In the present invention wear adjustment is provided by incorporation into the bridge of tandem automatic adjustment worm drives.
Worm gear sets have the advantages of providing high mechanical advantage in a compact space, and being designable to be “back-drive proof” (i.e., inhibiting undesired reverse rotation) by controlling the lead angle and the pressure angle of the worm threads and the coefficient of friction of the worm drive materials.
In one embodiment of the present invention's simpler and less-costly arrangements, there is provided a worm shaft with a ratchet gear arranged on the bridge with worm gear threads in the regions at or near the opposing ends of the worm shaft. The two sets of worm threads engage corresponding external teeth of the worm wheels (also referred to herein as brake application tappet wheels) such that rotation of the worm shaft results in the two brake application tappets advancing or retracting the tappets simultaneously. The threads may be all right- or left-handed, eliminating the need for separate left and right tappet designs. Alternatively, the threads on each side may be opposite-handed to cancel out each other's axial load in the shaft. The thread form may be trapezoidal.
Preferably, the worm shaft extends laterally across the bridge approximately perpendicular to the axes of rotation of the brake application tappets and parallel to the rotation axis of the rotary lever. Alternative orientations include the worm shaft being located diagonally in the bridge, such that the worm at one end of the shaft engages the teeth of one worm wheel on an upper side of the tappet, and the worm at the other end of the shaft engages the teeth of the other worm wheel on a lower side of the other tappet. The worm shaft may be located on an outer surface of the bridge, or may be located within the bridge, for example, captured by a cover plate of a two-piece bridge.
Alternatively, the ends of the shaft may incorporate any motion transfer arrangement that causes the brake application tappets to advance when the shaft is rotated, such as bevel gears that engage corresponding bevel gear teeth of the brake application tappets.
The rotation of the worm shaft is driven by the ratchet wheel (either attached to, or integrally formed with, the worm shaft). The ratchet wheel in turn is driven on the brake release stroke of the rotary lever. Preferably, the ratchet wheel is driven to rotate by a pawl provided tangentially to the ratchet wheel, between a brake disc-facing side of the rotary lever and the bridge. The pawl may be biased by a spring having a first end supported on the bridge. A second end of the spring is arranged to bias the pawl against the ratchet gear teeth. The spring may be, for example, linear, or torsional and concentric with the shaft. When the brake is released, the energy stored in the spring by the advancing rotary lever during the brake application stroke returns the pawl to its rest position as the rotary lever retracts.
The pawl is configured to ride over the unidirectional teeth of the ratchet wheel as the rotary lever advances in the brake application direction. Conversely, when the rotary lever moves in the brake release direction and a wear adjustment is needed to reduce the resting gap between the brake pads and the brake disc, a tooth on the pawl may engage a tooth on the ratchet wheel to cause the ratchet wheel to rotate. The rotation of the ratchet wheel causes the worm shaft to rotate. The rotation of the shaft's worms rotates a worm wheel positioned in the bridge around each brake application tappet present. Each worm wheel has internal threads which engage corresponding external threads of its respective tappet. Preferably the tappets are held against rotation while being supported on the bridge, so that when the tappet worm wheels are rotated, their internal threads drive the tappet external threads to axially advance the tappets toward the brake disc, thereby adjusting the lengths of the brake application tappets relative to the bridge to compensate for brake pad wear.
The amount of relative motion between the pawl and the ratchet gear in both the brake application and brake release directions is relatively small, and only occurs when brake adjustment is needed. Accordingly, the pawl teeth, ratchet wheel teeth, worm threads, and tappet threads may be sized such that the motion of the pawl in the brake release direction provides a desired amount of brake pad-to-brake disc gap reduction to compensate for brake pad wear, without the need for additional intermediate reduction gearing.
In a particularly preferred embodiment, the ratchet wheel is located at least partially in a slot in the bridge that is perpendicular to the rotation axis of the worm shaft. The slot is preferably axially wider than that the ratchet wheel, and includes a lateral contact surface configured to abut a corresponding surface of the ratchet wheel. With such a configuration, the ratchet wheel may be biased against the slot contact surface when the worn brake is applied so that friction inhibits undesired reverse rotation of the shaft (for example undesired reverse rotation resulting from vibrations experienced by the brake during road use). Then, during the release of the brake from a brake-applied state, the shaft is axially displaced by the axial force generated by interaction of the gears at the end of the shaft with their corresponding tappet worm wheels, thereby moving the ratchet wheel axially off of the slot contact surface to permit free rotation of the ratchet wheel in response to pawl engagement.
During brake application, the axial loading on the tappet causes friction between the threads of the tappets and the threads of their worm wheels to effectively lock the worm wheels against rotation in the bridge. With the worm gears unable to rotate, the shaft and the ratchet wheel accordingly are prevented from rotating, eliminating the potential for these components to rotate in an unwanted manner in response to stimuli such as vibrations during a brake application event. Further, during periods in which the brake is at rest (i.e., not applied), the pawl tooth that previously actuated the ratchet wheel remains in a position that prevents the ratchet wheel from rotating and altering the desired adjusted position of the tappets.
The brake components (e.g., pawl, rotary lever, bridge) are configured such that brake pad wear adjustments are only made when the rotary lever over-extends, i.e., passes a predetermined desired maximum rotary lever displacement due to brake wear.
In a further embodiment, at least one end of the shaft incorporates a wrenching feature (for example, a hex-shape) for manual adjustment of the brake during maintenance, for example for manual retraction of the brake application tappets to allow insertion of new brake pads and to reset the adjustment mechanism. The wrenching feature may be accessible via a normally-capped opening in the brake caliper housing. During such an adjustment, the pawl may be retracted so as not to engage the ratchet gear teeth.
The present invention provides multiple advantages by combining the functions of brake adjustment, adjustment locking, and tappet synchronization into one worm-geared shaft, which also reducing component complexity and costs.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.